You provide no information about the problem. It does not tune, is like a car does not start.

You need to provide some information about the problem.

Usually reading the users manual and performing the complete setup according it helps. (see above bar for the manuals) It is not use of tuning the gimbal if the basic setup is not properly performed.

If the basic setup and gimbal are ok, auto tuning does normally good job (assuming you have BaseCam 32 bit board with resent FW)

"The 5206 is the perfect size for cameras around 600-1500 grams" So that should not be your problem. Bigger motors could be better, but those should work. (if they provide mechanically enough support, depends of the gimbal construction, see roll axis below)

How is the gimbal mechanically, is it solid, all axis move freely, well balanced? If not, that is probably your problem.

Is this your gimbal http://www.hobbyking.com/hobbyking/store/__51496__Turnigy_PRO_Steady_Hand_Gimbal_3_Axis_KIT.html

if so, make it as small as posible, that helps, but do not expect marvels. It should be possible to get it working, though if you build it well and balance it well. My biggest worry would be the roll axis support, the pictures do not show that, is the roll axis well supported?